1. Field of the Invention
The present invention relates generally to a device for guiding and spreading a moving web of material. More particularly, the present invention relates to such a device wherein deviations in the direction of movement of the material web are corrected.
2. Description of Prior Art
A device of this type is disclosed in German published patent disclosure DE-OS No. 3,212,176 (FIGS. 4 and 5). In this German patent application the material web is placed between two spaced-apart fixed rolls in a 180.degree. loop across two guide rolls of the guide station supported in a rotary frame, wherein the downstream guide roll is followed by a spreading roll, which is pressed against the bottom side of the material web in the section of the web extending from the guide roll to the down stream fixing roll. The spreading roll is movable parallel with its axis and parallel with the fixing roll, and is forced against the material web by its own weight and by its drive. The edge-sensing system is arranged between the guide station and the spreading roll. The spreading effect of the spreading roll is adequate as long as the material web is running on the desired course. However, if the material web runs off-course, the web placed in the loop of the web across the guide station cannot be corrected to the desired extent because when running off-course, the segment of the web extending from the last guide roll to the fixing roll is distorted by the correction movement of the guide frame, and stretched significantly more along the edge disposed on the off-course side than the segment of the web on the opposite side. At the same time, due to the distortion on the off-course side, this segment of the web is lifted from the spreading roll, or at least relieved therefrom to such an extent that the spreading effect is negated on this side of the web, and lengthwise folds are formed in this segment near the edge due to increased tension. The web is greatly stressed mechanically when subjected to such distortion. The folds which result on the off-course side are undesirable in that they falsely "ionform" the edge sensor located on the off-course side, that the edge of the web is in a position conforming to the desired alignment, whereupon the edge sensor will prematurely interfere with the correcting action. Furthermore, a particularly detrimental effect is that the web deflected on the guide rolls is not very effectively supported thereby. Thus, the reactive force resulting from the motion by which the guide station distorts the segment up to the fixing roll as it is running off the guide station, whereby the latter, furthermore attempts to shift the edge of the web opposite the off-course side on the gripping surface of the spreading roll in the correcting direction, may cause slippage of the web on the downstream guide roll towards the off-course side, and thereby delay correcton. Consequently, due to the fact that the formation of folds on the off-course side causes the edge sensor on that side to prematurely interfere with the correction action, and that such correction is sluggish to begin with, the web will only slowly return to the desired alignment. Undesirable slow oscillation cannot be suppressed at high web speeds. Finally, a relatively large amount of space is required to accommodate the guide station, and due to the required loop of the web, a great length of unused web is also needed.
In another device of this type disclosed in German Patent DE-AS No. 1,499,060, there is no provision made for a spreading roll in connection with a web guiding device. In this device, the guide rolls are seated with both of their ends on swinging frames of the device via costly joint connections, with one of such frames being provided for each roll. This frame is engaged by adjustment cylinders for adjusting the angular position of at least one guide roll relative to the web, and thereby the looping around the roll.
In another guiding and spreading device, which is disclosed in British Patent Specification No. 1,186,039, three finger-like spreading rolls are disposed on each side of the web and supported in such a way that their angle relative to the edge of the web and thus their looping by the web can be individually adjusted on each side. The spreading rolls have a dual function in that they both guide and spread. Both devices according to the German and British disclosures do not permit high web speed. Furthermore, they have a spreading and guiding influence only on the extended edge zones of the web.
It is, therefore, the object of the present invention to provide an improved device of the type described above, whereby the material web is flawlessly guided and spread even at high web running speeds.